AIRBORNE INTERNET

AIRBORNE INTERNET

AIRBORNE INTERNET

1. INTRODUCTIONAirborne Internet is a concept that overlays computer network theory and principles into the transportation realm. The goal is to create information connectivity by providing a general purpose, multi-application data channel for people in transit. It is an approach to provide a general purpose high speed digital network to aviation. In doing so it has the potential to provide significant cost savings for aircrafts operators and the FAA, as it allows the consolidation of many functions into a common data channel. Numerous applications can use the same data channel. It gets its name from the fact that it works like the real internet.Airborne Internet began as a supporting technology for NASA’s Small Aircrafts Transportation System. But there is no reason that A.I should be limited to SATS-class aircraft. All of aviation, and even transportation, has the potential to benefit from A.I. Airborne Internet provide a general purpose data channel that numerous applications can use. By combining application and data functionality over a common data channel, aviation has the potential to significantly reduce costs for equipage on the ground and in the aircraft.The demand for Internet services is exploding and this creates a strong demand for broadband, high data rate service. It is expected that there will soon be a worldwide demand for Internet service in the hundreds of millions. The growth in use of the World Wide Web and electronic commerce will stimulate demand for broadband services.Airborne Internet is able to provide aircraft to ground, ground to ground and aircraft to aircraft communications in support of air traffic management, fleet operations, and passenger support services. Air transport aircraft could not only use A.I. for their own purposes, but they could provide a network router function that could sell excess bandwidth to other less bandwidth-demanding aircraft. This network in the sky not only reduces equipage and saves system costs, it could create a revenue stream for air carriers that does not currently exist. Many other applications can utilize the same A.I. data channel. The applications available are only limited by the bandwidth available.

2. WORLD OF AIRBORNE INTERNET2.1 WHY AIRBORNE INTERNET?There are mainly two reasons for the development of Airborne Internet. They are,2.1.1 SMALL AIRCRAFTS TRANSPORTATION SYSTEM:The first reason for the development of A.I is SATS. It began as a supporting technology for the NASA’s SATS. NASA is creating an infrastructure for fleets of small aircraft. People won’t have to fly between large cities on jet airliners. Instead, they will be able to fly themselves right to where they want to go. This would speed up air travel. But, it would need a major change in air traffic control to be able to manage thousands of small airplanes filling the skies. That’s where the “Airborne Internet” comes in. This project is being developed along with the Small Aircraft Transportation System (SATS). The SATS is studying the possibility of a system of 2- to 10-passenger airplanes. People could fly these small airplanes to and from small community or neighbourhood airports. Before this system becomes a reality, there are still many bugs that need to be worked out. Communication is one of the problems that will have to be fixed. The SATS would lead to thousands of inexperienced pilots flying airplanes. They would be flying to and from small airports that don’t usually have much traffic. Without major changes in air traffic control, the chances of plane crashes would greatly increase. That’s why NASA is developing the Airborne Internet.When people travel, they experience “connectivity down time” in which they are detached from the information that their network provided. Wireless networks are rapidly emerging to help fill this void. People that travel with laptops or personal digital assistants can obtain short term network connectivity from a business establishment when they stop for a break. Airport terminals are becoming popular “hot spots’ for wireless connectivity as people have time before and between flights to connect to the wireless network. The “human connectivity imperative” shows us a glaring absence of network connectivity during travel. While in motion on an aircraft, for example, people again lose the ability to connect. We design transportation systems to interconnect to complimentary forms of transportation. But these designs have ignored the information connectivity needs of the people who use it. The time people spend in transit could be turned into more productive time if network connectivity were available. This can be accomplished using the A.I.2.1.2 NEED FOR A HIGHER BANDWIDTH:The second reason is related with the need for a higher bandwidth. The computer most people use comes with a standard 56K modem, which means that in an ideal situation the computer would downstream at a rate of 56 kilobits per second (Kbps). That speed is far too slow to handle the huge streaming-video and music files that more consumers are demanding today. That's where the need for bigger bandwidth – broadband -- comes in, allowing a greater amount of data to flow to and from the computer. Land-based lines are limited physically in how much data they can deliver because of the diameter of the cable or phone line. In an airborne Internet, there is no such physical limitation, enabling a broader capacity.2.2 COMPARISON BETWEEN A.I & INTERNETTable 2.1 Comparison of A.I and Internet

Parameters

Internet

A.I

1. Distance Of Communication

2. Line Of Sight Obstruction

3. Antenna weight

4. Bandwidth

5. Delay

LowNot PresentCan be highComparatively HighNot Significant

Very HighPresentMust be lowBroadNot Significant

2.2 PRINCIPLE & WORKINGThe principle behind the A.I. is to establish a robust, reliable, and available digital data channel to aircraft. Establishing the general purpose, multi-application digital data channel connection to the aircraft is analogous to the connection of a desktop computer to its local area network, or even the wide area network we call the Internet. But aircraft are mobile objects. Therefore, mobile routing is required to maintain the data channel connectivity while the aircraft moves from region to region Mobile routing is the ability of a network user to move from one network to another without losing network connectivity. It has been developed and has matured to the point that it is ready to be applied to aviation.The current internet protocol (IP) is being replaced with a new version that includes provisions for security and mobile routing. It is specifically designed to accommodate the proliferation of wireless network devices that are easily transportable between networks. XML services, a standard way in which software interacts provide the opportunity for all information to be published as soon as it is available. This means the end user has the opportunity to receive near real time data, depending on the situation. XML is independent of the platform, operating system, or the device of the information source and the end user. Currently in aviation, very little information can be updated digitally during flight. At best, some information is updated using the analogue voice channel. Using XML aviation services, aircraft operators could receive automatic updates of weather, landing conditions at the destination airport, turbulence ahead, and other information. Airborne Internet could be the means by which the aviation industry will realize these benefits by providing XML services capability to aircraft.The A.I Aircraft will house packet switching circuitry and fast digital network functions. The communications antenna and related components will be located in a pod suspended below the aircraft fuselage. To offer "ubiquitous" service throughout a large region, the antenna will utilize multiple beams arranged in a typical cellular pattern. Broadband channels to subscribers in adjacent cells will be separated in frequency. As the beams traverse over a user location, the virtual path through the packet switch will be changed to perform a beam-to-beam handoff.The airborne Internet won't be completely wireless. There will be ground-based components to any type of airborne Internet network. The consumers will have to install an antenna on their home or business in order to receive signals from the network hub overhead. The networks will also work with established Internet Service Providers (ISPs), who will provide their high-capacity terminals for use by the network. These ISPs have a fibre point of presence -- their fibre optics is already set up. What the airborne Internet will do is provide an infrastructure that can reach areas that don't have broadband cables and wires.The Airborne Network will offer ubiquitous access to any subscriber within a "super metropolitan area" from an aircraft operating at high altitude. The aircraft will serve as the hub of the Airborne Network serving tens to hundreds of thousands of subscribers. Each subscriber will be able to communicate at multi-megabit per second data rates through a simple-to-install subscriber unit. The Airborne Network will be steadily evolved at a pace with the emergence of data communications technology world-wide. The Airborne Network will be a universal wireless communications network solution. It will be deployed globally on a city-by-city basis.An airplane specially designed for high altitude flight with a payload capacity of approximately one ton is being developed for commercial wireless services. It will circle at high altitudes for extended periods of time and it will serve as a stable platform from which broadband communications services will be offered. The High Altitude Long Operation (HALO) Aircraft will maintain station at an altitude of 52 to 60 thousand feet by flying in a circle with a diameter of about 5 to 8 nautical miles. Three successive shifts on station of 8 hours each can provide continuous coverage of an area for 24 hours per day, 7 days per week. Such a system can provide broadband multimedia communications to the general public.One such platform will cover an area of approximately 2800 square miles encompassing a typical metropolitan area. A viewing angle of 20 degrees or higher will be chosen to facilitate good line-of-sight coverage at millimeter wave (MMW) frequencies (20 GHz or higher). Operation at MMW frequencies enables broadband systems to be realized, i.e., from spectrum bandwidths of 1 to 6 GHz. MMW systems also permit very narrow beamwidths to be realized with small aperture antennas. Furthermore, since the aircraft is above most of the earth's oxygen, links to satellite constellations can be implemented using the frequencies overlapping the 60 GHz absorption band for good immunity from ground-based interference and good isolation from inter-satellite links.The A.I Network can utilize a cellular pattern on the ground so that each cell uses one of four frequency sub-bands, each having a bandwidth up to 60 MHz each way. A fifth sub-band can be used for gateways (connections to the public network or dedicated users). Each cell will cover an area of a few square miles. The entire bandwidth will be reused many times to achieve total coverage throughout the 2800 square mile area served by the airborne platform. The total capacity of the network supported by a single airborne platform can be greater than 100 Gbps. This is comparable to terrestrial fiber-optic (FO) networks and can provide two-way broadband multimedia services normally available only via FO networksThe Airborne Network provides an alternative to satellite- and ground-based systems. Unlike satellite systems, however, the airborne system concentrates all of the spectrum usage in certain geographic areas, which minimizes frequency coordination problems and permits sharing of frequency with ground-based systems. Enough power is available from the aircraft power generator to allow broadband data access from small user terminals2.3 SERVICE CLASSESThere are various classes of service to be provided. A consumer service would provide 1-5 Mbps communication links. A business service would provide 5-12.5 Mbps links. Since the links would be "bandwidth-on-demand," the total available spectrum would be time-shared between the various active sessions. The nominal data rates would be low while the peak rates would expand to a specified level. A gateway service can be provided for "dedicated" links of 25-155 Mbps2.4 DESIGN OPTIONS FOR ANTENNASThe Airborne Network will use an array of narrow beam antennas on the Airborne Aircraft to form multiple cells on the ground. Each cell covers a small geographic area, e.g., 4 to 8 square miles. The payload is liquid-cooled and operates off of about 20 kilowatts of DC power. An 18-foot dish underneath the plane is responsible for reflecting high-speed data signals from a ground station to our computer. The wide bandwidths and narrow beamwidths within each beam or cell are achieved by using MMW frequencies. Small aperture antennas can be used to achieve small cells. For example, an antenna having a diameter of only one foot can provide a beamwidth of less than three degrees. One hundred dish antennas can be easily carried by the Airborne Aircraft to create one hundred or more cells throughout the service area. If lensed antennas are utilized, wider beams can be created by combining beams through each lens aperture, and with multiple feeds behind each lens multiple beams can be formed by each compound lens.The major design options for antennas in the Communications Payload are to utilize either platform-fixed beams or earth-fixed beams.2.4.1 PLATFORM-FIXED BEAMS:For the case of platform-fixed beams, each antenna would have a fixed field of view. The total field of view for the entire Airborne Network would be the sum of these fields of view of the individual antennas. The network could initially have a small footprint and as demands on the Airborne services increase, additional antennas could be added to the communications Payload. This results in a modular design, readily adaptable for growth. Platform-fixed beams are simpler to construct generally, but require the "handoffs" between beams to be accomplished by the packet switching equipment as the beams "sweep" across the ground with the movement of the aircraft. However, the cost and performance penalties for frequently changing the virtual path through the packet switch may be appreciable.2.4.2 EARTH-FIXED BEAMS:An alternative is to electronically steer the beams so they remain "fixed" on the ground as the aircraft moves. This results in more electronic and physical complexity for the antennas, but this may be a good trade-off to make since the burden on the packet switch and its network management software would be greatly reduced. These trade-offs are still being assessed.For the case of earth-fixed beams, each antenna would have a wider field of view than the sum of the beams in that antenna since each beam can be steered in all directions. Each beam could be capable of steering throughout the Airborne footprint, or could be assigned a smaller portion. If there are "gaps" in the required coverage due to such things as rivers, hills, or forests, then the earth-fixed beams can be steered away from these undesirable coverage zones and more efficient usage of the antennas might result compared to the case of platform-fixed beams.A.I. could open up a whole new set of operating capabilities, cost savings, safety and efficiency for tomorrow’s aviation industry. The functions provided today that require the use of multiple on-board systems could be reduced to two simple systems.First, a rigorous and dependable method to maintain the airplane’s connection to the ground-based IP network is needed. This function is feasible using a combination of VHF radio (as is used for today’s aircraft communications) and an alternate, backup communication method. A satellite communication system could be employed for aircraft that fly in sparsely populated areas that are beyond VHF coverage of the existing NAS infrastructure, or for any aircraft that might lose VHF coverage (even temporarily). Satellite communication is currently being used for trans-oceanic flight today in which aircraft are clearly beyond range of the VHF radio system in the NAS.Second, a means of accurately determining an aircraft’s position is required. Current technology in GPS receivers provides position information reliably and accurately.It is possible that enough aircraft could utilize the A.I. architecture to create a virtual network in the sky. At any given moment, there are between 4500 and 6000 aircraft in flight over the United States.2.5 NETWORK ARCHITECTUREA critical first step in attaining the desirable capabilities of an Airborne Internet is a well-conceived architecture. Aircraft and landing facilities will be interconnected nodes in a high-speed digital communications network providing instant identification and information services on demand with seamless linking to the global transportation system. The Airborne Internet will leverage open standards and protocols for client-server network system architecture that are in development in the telecommunications industry for increased bandwidth for mobile applications.The secret to how well the Internet works is that it is a distributed network. In a centralized network, all computers are connected to one main server. They compete with each other to use that server. In the Internet, however, there is no central server. Content is stored on millions of computers around the world. And, the information can be accessed by millions more. Routers connect Internet users with what they are seeking. This creates a network that runs better because of the speed of millions of computers working together. A similar system would run the Airborne Internet. It would be a high-speed digital network. Information would be passed between aircrafts and the ground by the Internet. The aircrafts and the ground facilities would be the nodes in the network.Figure.2. Block Diagram Representation of Airborne Internet Network ArchitectureAt the apex of a wireless Cone of Commerce, the payload of the Airborne Aircraft becomes the hub of a star topology network for routing data packets between any two subscribers possessing premise equipment within the service coverage area. A single hop with only two links is required, each link connecting the payload to a subscriber. The links are wireless, broadband and line of sightInformation created outside the service area is delivered to the subscriber's consumer premise equipment ("CPE") through business premise equipment ("BPE") operated by Internet Service Providers ("ISPs") or content providers within that region, and through the Airborne Gateway ("HG") equipment directly connected to distant metropolitan areas via leased trunks. The HG is a portal serving the entire network. It avails system-wide access to content providers and it allows any subscriber to extend their communications beyond the Airborne Network service area by connecting them to dedicated long-distance lines such as inter-metro optical fiber.The CPE, BPE and HG all perform the same functions: use a high-gain antenna that automatically tracks the Airborne Aircraft; extract modulated signals conveyed through the air by millimeter waves; convert the extracted signals to digital data; provide standards-based data communications interfaces; and route the digital data to information appliances, personal computers, and workstations connected to the premise equipment. Thus, some of the technologies and components, both hardware and software, will be common to the designs of these three basic network elements

Figure3. The Airborne Network ArchitectureThe CPE, BPE and HG differ in size, complexity and cost, ranging from the CPE which is the smallest, least complex, lowest priced and will be expressively built for the mass market; followed by the BPE, engineered for a medium size business to provide access to multiple telecommuters by extending the corporate data communications network; to the HG which provides high bandwidth wireless data trunking to Wide Area Networks ("WANs") maintained and operated by the long distance carriers and content handlers who wish to distribute their products widely.In other words, the CPE is a personal gateway serving the consumer. The BPE is a gateway for the business requiring higher data rates. The HG, as a major element of the entire network, will be engineered to serve reliably as a critical network element. All of these elements are being demonstrated in related forms by terrestrial 38 GHz and LMDS vendors.

3. FEATURES & ADVANTAGES3.1 FEATURESThe key features of the Airborne Internet Network are summarized below. • Seamless ubiquitous multimedia services. • Adaptation to end user environments. • Enhanced user connectivity globally. • Rapidly deployable to sites of opportunity. • Secure and reliable information transactions. • Bandwidth on demand provides efficient use of available spectrum. • It helps to avoid the connectivity down time of people in transit. • It helps to achieve a broader bandwidth. • It has the potential to provide cost savings for aircrafts operators.

3.2 ADVANTAGESThe airborne Internet will function much like satellite-based Internet access, but without the time delay. Bandwidth of satellite and airborne Internet access are typically the same, but it will take less time for the airborne Internet to relay data because it is not as high up. Satellites orbit at several hundreds of miles above Earth. The airborne-Internet aircraft will circle overhead at an altitude of 52,000 to 69,000 feet (15,849 to 21,031 meters). At this altitude, the aircraft will be undisturbed by inclement weather and flying well above commercial air traffic.Networks using high-altitude aircraft will also have a cost advantage over satellites because the aircraft can be deployed easily -- they don't have to be launched into space. However, the airborne Internet will actually be used to compliment the satellite and ground-based networks, not replace them.These airborne networks will overcome the last-mile barriers facing conventional Internet access options. The "last mile" refers to the fact that access to high-speed cables still depends on physical proximity, and that for this reason, not everyone who wants access can have it. It would take a lot oftime to provide universal access using cable or phone lines, just because of the time it takes to install the wires. An airborne network will immediately overcome the last mile as soon as the aircraft takes off.The time people spend in transit could be turned into more productive time if network connectivity were available.It would be a high-speed digital networkIt has the potential to provide significant cost savings for aircrafts operators and the FAA, as it allows the consolidation of many functions into a common data channel.Numerous applications can use the same data channel.Since the Aircraft are operated from regional airports, the equipment will be routinely maintained and calibrated. This also allows for equipment upgrades as technology advances yield lower cost and weight and provide increased performance.4. APPLICATIONSSince the Airborne Internet provides broad band services, it increases the speed of downloading & uploading of data through it.A primary application for A.I. is to track aircraft for the air traffic control system. Aircraft pilots would let the traffic controllers know where they are through the network. The network would give the crew information that would help them avoid collisions. It would also allow information to be sent from aircraft to aircraft without having to go through ground facilities. The system could also be used to send safety warnings to aircraft.It has the potential to provide significant cost savings for aircrafts operators and the FAA, as it allows the consolidation of many functions into a common data channel. Numerous applications can use the same data channelUsing XML aviation services, aircraft operators could receive automatic updates of weather, landing conditions at the destination airport, turbulence ahead, and other information. Airborne Internet could be the means by which the aviation industry will realize these benefits by providing XML services capability to aircraft.

5. CONCLUSIONAI is the most recent development in the conventional internet of today. It takes the internet into transportation realms. It would be a high-speed digital network. Information would be passed between aircrafts and the ground by the Internet. Development of the Airborne Internet has already begun.Mainly three companies are planning to provide high-speed wireless Internet connection by placing aircraft in fixed patterns over hundreds of cities.Angel Technologies is planning an airborne Internet network, called High Altitude Long Operation (HALO), which would use lightweight planes to circle overhead and provide data delivery faster than a T1 line for businesses. Consumers would get a connection comparable to DSL. The centrepiece of this network is the Proteus plane, which will carry wireless networking equipment into the air. Each city in the HALO Network will be allotted three piloted Proteus planes. Each plane will fly for eight hours before the next plane takes off. After takeoff, the Proteus plane will climb to a safe altitude, above any bad weather or commercial traffic, and begin an 8-mile loop around the city. Each plane will accommodate two pilots, who will split flying duties during their eight-hour flight.NASA and AeroVironment are working on a solar-powered, lightweight plane that could fly over a city for six months or more, at 60,000 feet, without landing. AeroVironment plans to use these unmanned planes as the carrier to provide broadband Internet access. Helios is currently in the prototype stage.Sky Station International is counting on its blimps to deliver high-speed Internet access from high altitudes. Sky Station calls its blimps lighter-than-air platforms, and plans to station these airships over at least 250 cities worldwide, one over each city. Each station would fly at an altitude of 13 miles (21 km) and provide wireless service to an area of approximately 7,500 square miles (19,000 square km).The Airborne Network is capable of providing high rate communications to users of multimedia and broadband services. The feasibility of this approach is reasonably assured due to the convergence of technological advancements. The key enabling technologies at hand include: • GaAs RF devices which operate at MMW frequencies • Asynchronous Transfer Mode (ATM)/Synchronous Optical Network (SONET) Technology and Components • Digital Signal Processing for Wideband Signals • Video Compression • Very Dense Memory Capacity • Aircraft Technology

These technologies are individually available, to a great extent, from commercial markets. The Airborne Network seeks to integrate these various technologies into a service of high utility to small and medium businesses and other multimedia consumers at a reasonable cost.Airborne Internet will overtake the conventional internet in the near future, that is sure.

REFERENCE 1. www.AirborneInternet.com 2. www.NASAexplores.com 3. Airborne Internet/Collaborative Information Environment article. 4. www.aerospace.nasa.gov/curevent/tgir/index.html 5. White Paper on Airborne Internet.APPENDIX 1A.1. LMDS (LOCAL MULTIPOINT DISTRIBUTION ACCESS)LMDS is a broadband wireless point-to-multipoint communication system operating above 20 GHz (depending on country of licensing) that can be used to provide digital two-way voice, data, Internet, and video servicesThe acronym LMDS is derived from the following:L (local)—denotes that propagation characteristics of signals in this frequency range limit the potential coverage area of a single cell site; ongoing field trials conducted in metropolitan centers place the range of an LMDS transmitter at up to 5 milesM (multipoint)—indicates that signals are transmitted in a point-to-multipoint or broadcast method; the wireless return path, from subscriber to the base station, is a point-to-point transmissionD (distribution)—refers to the distribution of signals, which may consist of simultaneous voice, data, Internet, and video trafficS (service)—implies the subscriber nature of the relationship between the operator and the customer; the services offered through an LMDS network are entirely dependent on the operator's choice of businessA.1.1 WHY LMDS?More recent advances in a point-to-multipoint technology offer service providers a method of providing high-capacity local access that is less capital-intensive than a wire line solution, faster to deploy than wire line, and able to offer a combination of applications.Benefits of LMDS can be summarized as follows: • lower entry and deployment costs • ease and speed of deployment (systems can be deployed rapidly with minimal disruption to the community and the environment) • fast realization of revenue (as a result of rapid deployment) • demand-based build out (scalable architecture employing open industry standards ensuring services and coverage areas can be easily expanded as customer demand warrants) • cost shift from fixed to variable components (with traditional wire line systems, most of the capital investment is in the infrastructure, while with LMDS a greater percentage of the investment is shifted to customer-premise equipment [CPE], which means an operator spends dollars only when a revenue paying customer signs on) • no stranded capital when customers churn • cost-effective network maintenance, management, and operating costs

A.1.2 NETWORK ARCHITECTUREVarious network architectures are possible within LMDS system design. The majority of system operators will be using point-to-multipoint wireless access designs although point-to-point systems and TV distribution systems can be provided within the LMDS system. It is expected that the LMDS services will be a combination of voice, video, and data. Therefore, both asynchronous transfer mode (ATM) and Internet protocol (IP) transport methodologies are practical when viewed within the larger telecommunications infrastructure system of a nation.The LMDS network architecture consists of primarily four parts: network operations center (NOC), fiber-based infrastructure, base station, and customer premise equipment.The NOC contains the network management system (NMS) equipment that manages large regions of the customer network. Multiple NOCs can be interconnected. The fiber-based infrastructure typically consists of synchronous optical network (SONET) optical carrier (OC)–12, OC–3, and DS–3 links; central-office (CO) equipment; ATM and IP switching systems; and interconnections with the Internet and public switched telephone networks (PSTN).If local switching is present, customers connected to the base station can communicate with one another without entering the fiber infrastructure. The base station is where the conversion from fibered infrastructure to wireless infrastructure occurs. Base station equipment includes the network interface for fiber termination; modulation and demodulation functions; and microwave transmission and reception equipment typically located atop a roof or a pole. Key functionalities which may not be present in different designs include local switching. This function implies that billing, channel access management, registration, and authentication occur locally within the base station.The alternative base-station architecture simply provides connection to the fiber infrastructure. This forces all traffic to terminate in ATM switches or CO equipment somewhere in the fiber infrastructure. In this scenario, if two customers connected to the same base station wish to communicate with each other, they do so at a centralized location. Billing, authentication, registration, and traffic-management functions are performed centrally.The customer-premise configurations vary widely from vendor to vendor. Primarily, all configurations will include outdoor mounted microwave equipment and indoor digital equipment providing modulation, demodulation, control, and customer-premise interface functionality. The CPE may attach to the network using time-division multiple access (TDMA), frequency-division multiple access (FDMA), or code-division multiple access (CDMA) methodologies. The customer premise interfaces will run the full range from digital signal, level 0 (DS0), plain old telephone service (POTS), 10BaseT, unstructured DS1, structured DS1, frame relay, ATM25, serial ATM over T1, DS–3, OC–3, and OC–1. The customer premise locations will range from large enterprises (e.g., office buildings, hospitals, campuses), in which the microwave equipment is shared between many users, to mall locations and residences, in which single offices requiring 10BaseT and/or two POTS lines will be connected.